There’s one big reason Democrats face disaster Tuesday: They’ve done almost nothing in the last two years to make anyone — left or right — want to vote for them.

In 2008, recall, they made grand promises — to their base and to the center. They raised hopes through the roof.

Once in power, they kept virtually none of those vows. Instead, they took a whole different path — one neither side liked. Now they’re reaping the whirlwind.

So what’s their game plan? Rinse and repeat.

That’s right: President Obama and fellow Dems are again promising “change” — after the elections.

They failed the first time. They will again.

Consider the ’08 sales pitch: To leftists, they swore they’d do all the things they’d long pushed but President George W. Bush prevented: Close Gitmo. End wiretapping. Milk the rich. Tax carbon emissions. Ditch “Don’t Ask, Don’t Tell.” Reform immigration.

To moderates, even some conservatives, they vowed to drain Washington’s ethical swamp. Transcend partisanship. Win in Afghanistan (the war from which they’d been telling us Iraq was a needless distraction). Neutralize enemies like Iran through diplomacy.

Most of all, they vowed to put Americans back to work.

Two years later, we’re still waiting. And Dems are still vowing.

The left’s upset about Gitmo? Relax, we’re working on it. Don’t Ask, Don’t Tell? “It’ll end on Obama’s watch.” Sock the wealthy with new taxes? As good as done — after Election Day. Immigration reform? Tops on the agenda.

Even liberal comic Jon Stewart had to laugh Wednesday when Obama tried to revise his ’08 slogan, “Yes, we can” — adding: “But it’s not going to happen overnight.”

Meanwhile, everyone — left, right andcenter — wants to know: What’s up with the economy?

Why is unemployment still near 10 percent? When will economic activity keep pace, at least, with population growth?

Obama’s response: Our policies are “leading us out of this mess.”

Leading, maybe — but never actually getting anywhere.

But Democrats did pass ObamaCare, a massive program that came from nowhere and sickens many on the right and left — the right, because it marks a radical turn toward government-run medicine; the left, because it’s not radical enough. Even those in the middle are wondering why their health plans will change, after Obama swore ad nauseam that if you like your current plan, you could keep it.

Dems managed one other feat: They blew $1 trillion on “stimulus” — with no noticeable return on investment. The mind-blowing outlays reminded everyone why they hate Washington: Without a care about runaway spending, taxes and deficits, Dems lavished billions on their public-sector pals, pet pork projects and other worthless items.

That Dems wouldn’t keep their ’08 promises was entirely predictable — just as they’re sure to break this year’s vows, too. Why? Because reality rules: To close Gitmo, you need a place for detainees. Taxing even “just” the rich might trigger a double-dip recession. End wiretapping, and you put the nation at risk. Many in the military like “Don’t Ask, Don’t Tell.” Even flawless diplomacy won’t reform Iran.

Democrats will have an even harder time making good if they’re hit hard Tuesday. Not only will they lack votes to pass bills, Obama may be handed a bigger role in setting his party’s course — and as an ideologue, he might hew to a more radical, even less politically viable, agenda.

True, the return of Republicans to Washington might give Dems someone to blame in 2012 for their failures. But promise-fail-blame ain’t much of a strategy.

Another option for Dems: Do what the country wants. That is, keep taxes low to boost the economy. Constrain government and spending. Tend to the Mexican border. Stand up to our enemies. Level with the far left.